RFC 1294 PROPOSED STANDARD LINK LAYER · FRAME RELAY · ENCAPSULATION
Multiprotocol Interconnect over Frame Relay
How routers and bridges wrap IP and other protocols in Frame Relay frames so they can share the same virtual circuits.
RFC EDITOR
Bradley, T., Brown, C., and A. Malis, "Multiprotocol Interconnect over Frame Relay", RFC 1294, DOI 10.17487/RFC1294, January 1992, <https://www.rfc-editor.org/info/rfc1294>.
BIBTEX
@misc{rfc1294,
series = {Request for Comments},
number = 1294,
howpublished = {RFC 1294},
publisher = {RFC Editor},
doi = {10.17487/RFC1294},
url = {https://www.rfc-editor.org/info/rfc1294},
author = {T. Bradley and C. Brown and A. Malis},
title = {{Multiprotocol Interconnect over Frame Relay}},
pagetotal = 28,
year = 1992,
month = jan,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Network Working Group T. Bradley
Request for Comments: 1294 C. Brown
Wellfleet Communications, Inc.
A. Malis
BBN Communications
January 1992
Multiprotocol Interconnect over Frame Relay
1. Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
2. Abstract
This memo describes an encapsulation method for carrying network
interconnect traffic over a Frame Relay backbone. It covers aspects
of both Bridging and Routing. Systems with the ability to transfer
both this encapsulation method, and others must have a priori
knowledge of which virtual circuits will carry which encapsulation
method and this encapsulation must only be used over virtual circuits
that have been explicitly configured for its use.
3. Acknowledgements
Comments and contributions from many sources, especially those from
Ray Samora of Proteon, Ken Rehbehn of Netrix Corporation, Fred Baker
and Charles Carvalho of Advanced Computer Communications and Mostafa
Sherif of AT&T have been incorporated into this document. Special
thanks to Dory Leifer of University of Michigan for his contributions
to the resolution of fragmentation issues. This document could not
have been completed without the expertise of the IP over Large Public
Data Networks working group of the IETF.
4. Conventions
The following language conventions are used in the items of
specification in this document:
o Must, Shall or Mandatory -- the item is an absolute
requirement of the specification.
o Should or Recommended -- the item should generally be
followed for all but exceptional circumstances.
o May or Optional -- the item is truly optional and may be
followed or ignored according to the needs of the
implementor.
5. Introduction
The following discussion applies to those devices which serve as end
stations (DTEs) on a public or private Frame Relay network (for
example, provided by a common carrier or PTT). It will not discuss
the behavior of those stations that are considered a part of the
Frame Relay network (DCEs) other than to explain situations in which
the DTE must react.
The Frame Relay network provides a number of virtual circuits that
form the basis for connections between stations attached to the same
Frame Relay network. The resulting set of interconnected devices
forms a private Frame Relay group which may be either fully
interconnected with a complete "mesh" of virtual circuits, or only
partially interconnected. In either case, each virtual circuit is
uniquely identified at each Frame Relay interface by a Data Link
Connection Identifier (DLCI). In most circumstances DLCIs have
strictly local significance at each Frame Relay interface.
The specifications in this document are intended to apply to both
switched and permanent virtual circuits.
6. Frame Format
All protocols must encapsulate their packets within a Q.922 Annex A
frame [1,2]. Additionally, frames shall contain information
necessary to identify the protocol carried within the Protocol Data
Unit (PDU), thus allowing the receiver to properly process the
incoming packet. The format shall be as follows:
+-----------------------------+
| flag (7E hexadecimal) |
+-----------------------------+
| Q.922 Address* |
+-- --+
| |
+-----------------------------+
| Control (UI = 0x03) |
+-----------------------------+
| Optional Pad(s) (0x00) |
+-----------------------------+
| NLPID |
+-----------------------------+
| . |
| . |
| . |
| Data |
| . |
| . |
+-----------------------------+
| Frame Check Sequence |
+-- . --+
| (two octets) |
+-----------------------------+
| flag (7E hexadecimal) |
+-----------------------------+
* Q.922 addresses, as presently defined, are two octets and
contain a 10-bit DLCI. In some networks Q.922 addresses may
optionally be increased to three or four octets.
The control field is the Q.922 control field. The UI (0x03) value is
used unless it is negotiated otherwise. The use of XID (0xAF or
0xBF) is permitted and is discussed later.
The pad field is an optional field used to align the remainder of the
frame to a convenient boundary for the sender. There may be zero or
more pad octets within the pad field and all must have a value of
zero.
The Network Level Protocol ID (NLPID) field is administered by ISO
and CCITT. It contains values for many different protocols including
IP, CLNP and IEEE Subnetwork Access Protocol (SNAP)[10]. This field
tells the receiver what encapsulation or what protocol follows.
Values for this field are defined in ISO/IEC TR 9577 [3]. A NLPID
value of 0x00 is defined within ISO/IEC TR 9577 as the Null Network
Layer or Inactive Set. Since it cannot be distinguished from a pad
field, and because it has no significance within the context of this
encapsulation scheme, a NLPID value of 0x00 is invalid under the
Frame Relay encapsulation. The known NLPID values are listed in the
Appendix.
For full interoperability with older Frame Relay encapsulation
formats, a station may implement section 15, Backward Compatibility.
There is no commonly implemented maximum frame size for Frame Relay.
A network must, however, support at least a 262 octet maximum.
Generally, the maximum will be greater than or equal to 1600 octets,
but each Frame Relay provider will specify an appropriate value for
its network. A Frame Relay DTE, therefore, must allow the maximum
acceptable frame size to be configurable.
The minimum frame size allowed for Frame Relay is five octets between
the opening and closing flags.
7. Interconnect Issues
There are two basic types of data packets that travel within the
Frame Relay network, routed packets and bridged packets. These
packets have distinct formats and therefore, must contain an
indication that the destination may use to correctly interpret the
contents of the frame. This indication is embedded within the NLPID
and SNAP header information.
For those protocols that do not have a NLPID already assigned, it is
necessary to provide a mechanism to allow easy protocol
identification. There is a NLPID value defined indicating the
presence of a SNAP header.
A SNAP header is of the form
+-------------------------------+
| Organizationally Unique |
+-- +---------------+
| Identifier | Protocol |
+---------------+---------------+
| Identifier |
+---------------+
All stations must be able to accept and properly interpret both the
NLPID encapsulation and the SNAP header encapsulation for a routed
packet.
The three-octet Organizationally Unique Identifier (OUI) identifies
an organization which administers the meaning of the Protocol
Identifier (PID) which follows. Together they identify a distinct
protocol. Note that OUI 0x00-00-00 specifies that the following PID
is an EtherType.
7.1. Routed Frames
Some protocols will have an assigned NLPID, but because the NLPID
numbering space is so limited many protocols do not have a specific
NLPID assigned to them. When packets of such protocols are routed
over Frame Relay networks they are sent using the NLPID 0x80 (which
indicates a SNAP follows), OUI 0x00-00-00 (which indicates an
EtherType follows), and the EtherType of the protocol in use.
Format of Routed Frames
+-------------------------------+
| Q.922 Address |
+-------------------------------+
|Control 0x03 | pad(s) 0x00 |
+-------------------------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+ --+
| OUI 0x00-00 |
+-------------------------------+
| EtherType |
+-------------------------------+
| Protocol Data |
+-------------------------------+
| FCS |
+-------------------------------+
In the few cases when a protocol has an assigned NLPID (see
appendix), 48 bits can be saved using the format below:
Format of Routed NLPID Protocol
+-------------------------------+
| Q.922 Address |
+-------------------------------+
|Control 0x03 | NLPID |
+-------------------------------+
| Protocol Data |
+-------------------------------+
| FCS |
+-------------------------------+
In the particular case of an Internet IP datagram, the NLPID is 0xCC.
Format of Routed IP Datagram
+-------------------------------+
| Q.922 Address |
+-------------------------------+
|Control 0x03 | NLPID 0xCC |
+-------------------------------+
| IP Datagram |
+-------------------------------+
| FCS |
+-------------------------------+
7.2. Bridged Frames
The second type of Frame Relay traffic is bridged packets. These
packets are encapsulated using the NLPID value of 0x80 indicating
SNAP and the following SNAP header identifies the format of the
bridged packet. The OUI value used for this encapsulation is the
802.1 organization code 0x00-80-C2. The following two octets (PID)
specify the form of the MAC header, which immediately follows the
SNAP header. Additionally, the PID indicates whether the original
FCS is preserved within the bridged frame.
The 802.1 organization has reserved the following values to be used
with Frame Relay:
PID Values for OUI 0x00-80-C2
with preserved FCS w/o preserved FCS Media
------------------ ----------------- ----------------
0x00-01 0x00-07 802.3/Ethernet
0x00-02 0x00-08 802.4
0x00-03 0x00-09 802.5
0x00-04 0x00-0A FDDI
0x00-05 0x00-0B 802.6
In addition, the PID value 0x00-0E, when used with OUI 0x00-80-C2,
identifies Bridged Protocol Data Units (BPDUs).
A packet bridged over Frame Relay will, therefore, have one of the
following formats:
Format of Bridged Ethernet/802.3 Frame
+-------------------------------+
| Q.922 Address |
+-------------------------------+
|Control 0x03 | pad(s) 0x00 |
+-------------------------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+ --+
| OUI 0x80-C2 |
+-------------------------------+
| PID 0x00-01 or 0x00-07 |
+-------------------------------+
| MAC destination address |
+-------------------------------+
| (remainder of MAC frame) |
+-------------------------------+
| LAN FCS (if PID is 0x00-01) |
+-------------------------------+
| FCS |
+-------------------------------+
Format of Bridged 802.4 Frame
+-------------------------------+
| Q.922 Address |
+-------------------------------+
|Control 0x03 | pad(s) 0x00 |
+-------------------------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+ --+
| OUI 0x80-C2 |
+-------------------------------+
| PID 0x00-02 or 0x00-08 |
+-------------------------------+
| pad 0x00 | Frame Control |
+-------------------------------+
| MAC destination address |
+-------------------------------+
| (remainder of MAC frame) |
+-------------------------------+
| LAN FCS (if PID is 0x00-02) |
+-------------------------------+
| FCS |
+-------------------------------+
Format of Bridged 802.5 Frame
+-------------------------------+
| Q.922 Address |
+-------------------------------+
|Control 0x03 | pad(s) 0x00 |
+-------------------------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+ --+
| OUI 0x80-C2 |
+-------------------------------+
| PID 0x00-03 or 0x00-09 |
+-------------------------------+
| Access Control| Frame Control |
+-------------------------------+
| MAC destination address |
| . |
| . |
+-------------------------------+
| (remainder of MAC frame) |
+-------------------------------+
| LAN FCS (if PID is 0x00-03) |
| |
+-------------------------------+
| FCS |
+-------------------------------+
Format of Bridged FDDI Frame
+-------------------------------+
| Q.922 Address |
+-------------------------------+
|Control 0x03 | pad(s) 0x00 |
+-------------------------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+ --+
| OUI 0x80-C2 |
+-------------------------------+
| PID 0x00-04 or 0x00-0A |
+-------------------------------+
| Access Control| Frame Control |
+-------------------------------+
| MAC destination address |
| . |
| . |
+-------------------------------+
| (remainder of MAC frame) |
+-------------------------------+
| LAN FCS (if PID is 0x00-04) |
| |
+-------------------------------+
| FCS |
+-------------------------------+
Format of Bridged 802.6 Frame
+-------------------------------+
| Q.922 Address |
| Control 0x03 | pad(s) 0x00 |
+-------------------------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+ --+
| OUI 0x80-C2 |
+-------------------------------+
| PID 0x00-05 or 0x00-0B |
+-------------------------------+
| Reserved | BEtag | Common
+---------------+---------------+ PDU
| BAsize | Header
+-------------------------------+
| MAC destination address |
+-------------------------------+
| (remainder of MAC frame) |
+-------------------------------+
| |
+- Common PDU Trailer -+
| |
+-------------------------------+
| FCS |
+-------------------------------+
The Common Protocol Data Unit (PDU) Header and Trailer are
conveyed to allow pipelining at the egress bridge to an 802.6
subnetwork. Specifically, the Common PDU Header contains the
BAsize field, which contains the length of the PDU. If this field
is not available to the egress 802.6 bridge, then that bridge
cannot begin to transmit the segmented PDU until it has received
the entire PDU, calculated the length, and inserted the length
into the BAsize field. If the field is available, the egress
802.6 bridge can extract the length from the BAsize field of the
Common PDU Header, insert it into the corresponding field of the
first segment, and immediately transmit the segment onto the 802.6
subnetwork. Thus, the bridge can begin transmitting the 802.6 PDU
before it has received the complete PDU.
One should note that the Common PDU Header and Trailer of the
encapsulated frame should not be simply copied to the outgoing
802.6 subnetwork because the encapsulated BEtag value may conflict
with the previous BEtag value transmitted by that bridge.
Format of BPDU Frame
+-------------------------------+
| Q.922 Address |
+-------------------------------+
|Control 0x03 | pad(s) 0x00 |
+-------------------------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+ --+
| OUI 0x80-C2 |
+-------------------------------+
| PID 0x00-0E |
+-------------------------------+ ----
| 802.1(d) Protocol Identifier | BPDU, as defined
+-------------------------------+ by 802.1(d),
| Version = 00 | BPDU Type | section 5.3
+-------------------------------+
| (remainder of BPDU) |
+-------------------------------+ ----
| FCS |
+-------------------------------+
8. Data Link Layer Parameter Negotiation
Frame Relay stations may choose to support the Exchange
Identification (XID) specified in Appendix III of Q.922 [1]. This
XID exchange allows the following parameters to be negotiated at the
initialization of a Frame Relay circuit: maximum frame size N201,
retransmission timer T200, and the maximum number of outstanding I
frames K.
A station may indicate its unwillingness to support acknowledged mode
multiple frame operation by specifying a value of zero for the
maximum window size, K.
If this exchange is not used, these values must be statically
configured by mutual agreement of Data Link Connection (DLC)
endpoints, or must be defaulted to the values specified in Section
5.9 of Q.922:
N201: 260 octets
K: 3 for a 16 Kbps link,
7 for a 64 Kbps link,
32 for a 384 Kbps link,
40 for a 1.536 Mbps or above link
T200: 1.5 seconds [see Q.922 for further details]
If a station supporting XID receives an XID frame, it shall respond
with an XID response. In processing an XID, if the remote maximum
frame size is smaller than the local maximum, the local system shall
reduce the maximum size it uses over this DLC to the remotely
specified value. Note that this shall be done before generating a
response XID.
The following diagram describes the use of XID to specify non-use of
acknowledged mode multiple frame operation.
Non-use of Acknowledged Mode Multiple Frame Operation
+---------------+
| Address | (2,3 or 4 octets)
| |
+---------------+
| Control 0xAF |
+---------------+
| format 0x82 |
+---------------+
| Group ID 0x80 |
+---------------+
| Group Length | (2 octets)
| 0x00-0E |
+---------------+
| 0x05 | PI = Frame Size (transmit)
+---------------+
| 0x02 | PL = 2
+---------------+
| Maximum | (2 octets)
| Frame Size |
+---------------+
| 0x06 | PI = Frame Size (receive)
+---------------+
| 0x02 | PL = 2
+---------------+
| Maximum | (2 octets)
| Frame Size |
+---------------+
| 0x07 | PI = Window Size
+---------------+
| 0x01 | PL = 1
+---------------+
| 0x00 |
+---------------+
| 0x09 | PI = Retransmission Timer
+---------------+
| 0x01 | PL = 1
+---------------+
| 0x00 |
+---------------+
| FCS | (2 octets)
| |
+---------------+
9. Fragmentation Issues
Fragmentation allows the exchange of packets that are greater than
the maximum frame size supported by the underlying network. In the
case of Frame Relay, the network may support a maximum frame size as
small as 262 octets. Because of this small maximum size, it is
advantageous to support fragmentation and reassembly.
Unlike IP fragmentation procedures, the scope of Frame Relay
fragmentation procedure is limited to the boundary (or DTEs) of the
Frame Relay network.
The general format of fragmented packets is the same as any other
encapsulated protocol. The most significant difference being that
the fragmented packet will contain the encapsulation header. That
is, a packet is first encapsulated (with the exception of the address
and control fields) as defined above. Large packets are then broken
up into frames appropriate for the given Frame Relay network and are
encapsulated using the Frame Relay fragmentation format. In this
way, a station receiving fragments may reassemble them and then put
the reassembled packet through the same processing path as a packet
that had not been fragmented.
Within Frame Relay fragments are encapsulated using the SNAP format
with an OUI of 0x00-80-C2 and a PID of 0x00-0D. Individual fragments
will, therefore, have the following format:
+---------------+---------------+
| Q.922 Address |
+---------------+---------------+
| Control 0x03 | pad 0x00 |
+---------------+---------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+---------------+
| OUI 0x80-C2 |
+---------------+---------------+
| PID 0x00-0D |
+---------------+---------------+
| sequence number |
+---------------+---------------+
|F| RSVD |offset |
+---------------+---------------+
| fragment data |
| . |
| . |
| . |
+---------------+---------------+
| FCS |
+---------------+---------------+
The sequence field is a two octet identifier that is incremented
every time a new complete message is fragmented. It allows detection
of lost frames and is set to a random value at initialization.
The reserved field is 4 bits long and is not currently defined. It
must be set to 0.
The final bit is a one bit field set to 1 on the last fragment and
set to 0 for all other fragments.
The offset field is an 11 bit value representing the logical offset
of this fragment in bytes divided by 32. The first fragment must have
an offset of zero.
The following figure shows how a large IP datagram is fragmented over
Frame Relay. In this example, the complete datagram is fragmented
into two Frame Relay frames.
Frame Relay Fragmentation Example
+-----------+-----------+
| Q.922 Address |
+-----------+-----------+
| Ctrl 0x03 | pad 0x00 |
+-----------+-----------+
|NLPID 0x80 | OUI 0x00 |
+-----------+-----------+
| OUI 0x80-C2 |
+-----------+-----------+ +-----------+-----------+
| pad 0x00 |NLPID 0xCC | | PID 0x00-0D |
+-----------+-----------+ +-----------+-----------+
| | | sequence number n |
| | +-----------+-----------+
| | |0| RSVD |offset (0) |
| | +-----------+-----------+
| | | pad 0x00 |NLPID 0xCC |
| | +-----------+-----------+
| | | first m bytes of |
| large IP datagram | ... | IP datagram |
| | | |
| | +-----------+-----------+
| | | FCS |
| | +-----------+-----------+
| |
| | +-----------+-----------+
| | | Q.922 Address |
| | +-----------+-----------+
| | | Ctrl 0x03 | pad 0x00 |
+-----------+-----------+ +-----------+-----------+
|NLPID 0x80 | OUI 0x00 |
+-----------+-----------+
| OUI 0x80-C2 |
+-----------+-----------+
| PID 0x00-0D |
+-----------+-----------+
| sequence number n |
+-----------+-----------+
|1| RSVD |offset (m/32) |
+-----------+-----------+
| remainder of IP |
| datagram |
+-----------+-----------+
| FCS |
+-----------+-----------+
Fragments must be sent in order starting with a zero offset and
ending with the final fragment. These fragments must not be
interrupted with other packets or information intended for the same
DLC. An end station must be able to re-assemble up to 2K octets and
is suggested to support up to 8K octet re-assembly. If at any time
during this re-assembly process, a fragment is corrupted or a
fragment is missing, the entire message is dropped. The upper layer
protocol is responsible for any retransmission in this case.
This fragmentation algorithm is not intended to reliably handle all
possible failure conditions. As with IP fragmentation, there is a
small possibility of reassembly error and delivery of an erroneous
packet. Inclusion of a higher layer checksum greatly reduces this
risk.
10. Address Resolution
There are situations in which a Frame Relay station may wish to
dynamically resolve a protocol address. Address resolution may be
accomplished using the standard Address Resolution Protocol (ARP) [6]
encapsulated within a SNAP encoded Frame Relay packet as follows:
+-----------------------+-----------------------+
| Q.922 Address |
+-----------------------+-----------------------+
| Control (UI) 0x03 | pad(s) 0x00 |
+-----------------------+-----------------------+
| NLPID = 0x80 | | SNAP Header
+-----------------------+ OUI = 0x00-00-00 + Indicating
| | ARP
+-----------------------+-----------------------+
| PID = 0x0806 |
+-----------------------+-----------------------+
| ARP packet |
| . |
| . |
| . |
+-----------------------+-----------------------+
Where the ARP packet has the following format and values:
Data:
ar$hrd 16 bits Hardware type
ar$pro 16 bits Protocol type
ar$hln 8 bits Octet length of hardware address (n)
ar$pln 8 bits Octet length of protocol address (m)
ar$op 16 bits Operation code (request or reply)
ar$sha noctets source hardware address
ar$spa moctets source protocol address
ar$tha noctets target hardware address
ar$tpa moctets target protocol address
ar$hrd - assigned to Frame Relay is 15 decimal
(0x000F) [7].
ar$pro - see assigned numbers for protocol ID number for
the protocol using ARP. (IP is 0x0800).
ar$hln - length in bytes of the address field (2, 3, or 4)
ar$pln - protocol address length is dependent on the
protocol (ar$pro) (for IP ar$pln is 4).
ar$op - 1 for request and 2 for reply.
ar$sha - Q.922 source hardware address, with C/R, FECN,
BECN, and DE set to zero.
ar$tha - Q.922 target hardware address, with C/R, FECN,
BECN, and DE set to zero.
Because DLCIs within most Frame Relay networks have only local
significance, an end station will not have a specific DLCI assigned
to itself. Therefore, such a station does not have an address to put
into the ARP request or reply. Fortunately, the Frame Relay network
does provide a method for obtaining the correct DLCIs. The solution
proposed for the locally addressed Frame Relay network below will
work equally well for a network where DLCIs have global significance.
The DLCI carried within the Frame Relay header is modified as it
traverses the network. When the packet arrives at its destination,
the DLCI has been set to the value that, from the standpoint of the
receiving station, corresponds to the sending station. For example,
in figure 1 below, if station A were to send a message to station B,
it would place DLCI 50 in the Frame Relay header. When station B
received this message, however, the DLCI would have been modified by
the network and would appear to B as DLCI 70.
~~~~~~~~~~~~~~~
( )
+-----+ ( ) +-----+
| |-50------(--------------------)---------70-| |
| A | ( ) | B |
| |-60-----(---------+ ) | |
+-----+ ( | ) +-----+
( | )
( | ) <---Frame Relay
~~~~~~~~~~~~~~~~ network
80
|
+-----+
| |
| C |
| |
+-----+
Figure 1
Lines between stations represent data link connections (DLCs).
The numbers indicate the local DLCI associated with each
connection.
DLCI to Q.922 Address Table for Figure 1
DLCI (decimal) Q.922 address (hex)
50 0x0C21
60 0x0CC1
70 0x1061
80 0x1401
If you know about frame relay, you should understand the
corrolation between DLCI and Q.922 address. For the uninitiated,
the translation between DLCI and Q.922 address is based on a two
byte address length using the Q.922 encoding format. The format
is:
8 7 6 5 4 3 2 1
+------------------------+---+--+
| DLCI (high order) |c/r|ea|
+------------------------+---+--+
| DLCI (lower) |FECN|BECN|DE |EA|
+--------------+----+----+---+--+
For ARP and its variants, the FECN, BECN, C/R and DE bits are
assumed to be 0.
When an ARP message reaches a destination, all hardware addresses
will be invalid. The address found in the frame header will,
however, be correct. Though it does violate the purity of layering,
Frame Relay may use the address in the header as the sender hardware
address. It should also be noted that the target hardware address,
in both ARP request and reply, will also be invalid. This should not
cause problems since ARP does not rely on these fields and in fact,
an implementation may zero fill or ignore the target hardware address
field entirely.
As an example of how this address replacement scheme may work, refer
to figure 1. If station A (protocol address pA) wished to resolve
the address of station B (protocol address pB), it would format an
ARP request with the following values:
ARP request from A
ar$op 1 (request)
ar$sha unknown
ar$spa pA
ar$tha undefined
ar$tpa pB
Because station A will not have a source address associated with it,
the source hardware address field is not valid. Therefore, when the
ARP packet is received, it must extract the correct address from the
Frame Relay header and place it in the source hardware address field.
This way, the ARP request from A will become:
ARP request from A as modified by B
ar$op 1 (request)
ar$sha 0x1061 (DLCI 70) from Frame Relay header
ar$spa pA
ar$tha undefined
ar$tpa pB
Station B's ARP will then be able to store station A's protocol
address and Q.922 address association correctly. Next, station B
will form a reply message. Many implementations simply place the
source addresses from the ARP request into the target addresses and
then fills in the source addresses with its addresses. In this case,
the ARP response would be:
ARP response from B
ar$op 2 (response)
ar$sha unknown
ar$spa pB
ar$tha 0x1061 (DLCI 70)
ar$tpa pA
Again, the source hardware address is unknown and when the request is
received, station A will extract the address from the Frame Relay
header and place it in the source hardware address field. Therefore,
the response will become:
ARP response from B as modified by A
ar$op 2 (response)
ar$sha 0x0C21 (DLCI 50)
ar$spa pB
ar$tha 0x1061 (DLCI 70)
ar$tpa pA
Station A will now correctly recognize station B having protocol
address pB associated with Q.922 address 0x0C21 (DLCI 50).
Reverse ARP (RARP) [8] will work in exactly the same way. Still
using figure 1, if we assume station C is an address server, the
following RARP exchanges will occur:
RARP request from A RARP request as modified by C
ar$op 3 (RARP request) ar$op 3 (RARP request)
ar$sha unknown ar$sha 0x1401 (DLCI 80)
ar$spa undefined ar$spa undefined
ar$tha 0x0CC1 (DLCI 60) ar$tha 0x0CC1 (DLCI 60)
ar$tpa pC ar$tpa pC
Station C will then look up the protocol address corresponding to
Q.922 address 0x1401 (DLCI 80) and send the RARP response.
RARP response from C RARP response as modified by A
ar$op 4 (RARP response) ar$op 4 (RARP response)
ar$sha unknown ar$sha 0x0CC1 (DLCI 60)
ar$spa pC ar$spa pC
ar$tha 0x1401 (DLCI 80) ar$tha 0x1401 (DLCI 80)
ar$tpa pA ar$tpa pA
This means that the Frame Relay interface must only intervene in the
processing of incoming packets.
In the absence of suitable multicast, ARP may still be implemented.
To do this, the end station simply sends a copy of the ARP request
through each relevant DLC, thereby simulating a broadcast.
The use of multicast addresses in a Frame Relay environment is
presently under study by Frame Relay providers. At such time that
the issues surrounding multicasting are resolved, multicast
addressing may become useful in sending ARP requests and other
"broadcast" messages.
Because of the inefficiencies of broadcasting in a Frame Relay
environment, a new address resolution variation was developed. It is
called Inverse ARP [11] and describes a method for resolving a
protocol address when the hardware address is already known. In
Frame Relay's case, the known hardware address is the DLCI. Using
Inverse ARP for Frame Relay follows the same pattern as ARP and RARP
use. That is the source hardware address is inserted at the
receiving station.
In our example, station A may use Inverse ARP to discover the
protocol address of the station associated with its DLCI 50. The
Inverse ARP request would be as follows:
InARP Request from A (DLCI 50)
ar$op 8 (InARP request)
ar$sha unknown
ar$spa pA
ar$tha 0x0C21 (DLCI 50)
ar$tpa unknown
When Station B receives this packet, it will modify the source
hardware address with the Q.922 address from the Frame Relay header.
This way, the InARP request from A will become:
ar$op 8 (InARP request)
ar$sha 0x1061
ar$spa pA
ar$tha 0x0C21
ar$tpa unknown.
Station B will format an Inverse ARP response and send it to station
A as it would for any ARP message.
11. IP over Frame Relay
Internet Protocol [9] (IP) datagrams sent over a Frame Relay network
conform to the encapsulation described previously. Within this
context, IP could be encapsulated in two different ways.
1. NLPID value indicating IP
+-----------------------+-----------------------+
| Q.922 Address |
+-----------------------+-----------------------+
| Control (UI) 0x03 | NLPID = 0xCC |
+-----------------------+-----------------------+
| IP Packet . |
| . |
| . |
+-----------------------+-----------------------+
2. NLPID value indicating SNAP
+-----------------------+-----------------------+
| Q.922 Address |
+-----------------------+-----------------------+
| Control (UI) 0x03 | pad(s) 0x00 |
+-----------------------+-----------------------+
| NLPID = 0x80 | | SNAP Header
+-----------------------+ OUI = 0x00-00-00 + Indicating
| | IP
+-----------------------+-----------------------+
| PID = 0x0800 |
+-----------------------+-----------------------+
| IP packet |
| . |
| . |
| . |
+-----------------------+-----------------------+
Although both of these encapsulations are supported under the given
definitions, it is advantageous to select only one method as the
appropriate mechanism for encapsulating IP data. Therefore, IP data
shall be encapsulated using the NLPID value of 0xCC indicating IP as
shown in option 1 above. This (option 1) is more efficient in
transmission (48 fewer bits), and is consistent with the
encapsulation of IP in X.25.
12. Other Protocols over Frame Relay
As with IP encapsulation, there are alternate ways to transmit
various protocols within the scope of this definition. To eliminate
the conflicts, the SNAP encapsulation is only used if no NLPID value
is defined for the given protocol.
As an example of how this works, ISO CLNP has a NLPID defined (0x81).
Therefore, the NLPID field will indicate ISO CLNP and the data packet
will follow immediately. The frame would be as follows:
+----------------------+----------------------+
| Q.922 Address |
+----------------------+----------------------+
| Control (0x03) | NLPID - 0x81 (CLNP) |
+---------------------------------------------+
| CLNP packet |
| . |
| . |
+---------------------------------------------+
13. Bridging in a Frame Relay network
A Frame Relay interface acting as a bridge must be able to flood,
forward, and filter packets.
Flooding is performed by sending the packet to all possible
destinations. In the Frame Relay environment this means sending the
packet through each relevant DLC.
To forward a packet, a bridge must be able to associate a destination
MAC address with a DLC. It is unreasonable and perhaps impossible to
require bridges to statically configure an association of every
possible destination MAC address with a DLC. Therefore, Frame Relay
bridges must provide enough information to allow a Frame Relay
interface to dynamically learn about foreign destinations beyond the
set of Frame Relay stations.
To accomplish dynamic learning, a bridged packet shall conform to the
encapsulation described within section 7. In this way, the receiving
Frame Relay interface will know to look into the bridged packet and
learn the association between foreign destination and Frame Relay
station.
14. For Future Study
It may be desirable for the two ends of a connection to have the
capability to negotiate end-to-end configuration and service
parameters. The actual protocol and parameters to be negotiated will
be a topic of future RFCs.
15. Backward Compatibility
This section is included in this RFC for completeness only. It is
not intended to suggest additional requirements.
Some existing Frame Relay stations use the NLPID value of 0xCE to
indicate an escape to Ethernet Packet Types as defined in the latest
version of the Assigned Numbers (RFC-1060) [7]. In this case, the
frame will have the following format:
+-----------------------------+
| Q.922 Address |
+-- --+
| |
+-----------------------------+
| Control (UI = 0x03) |
+-----------------------------+
| Optional Pad(s) (0x00) |
+-----------------------------+
| NLPID (0xCE) |
+-----------------------------+
| Ethertype |
+- -+
| |
+-----------------------------+
| . |
| . |
| Data |
| . |
| . |
+-----------------------------+
| Frame Check Sequence |
+-- . --+
| (two octets) |
+-----------------------------+
The Ethertype field is a 16-bit value used to identify a protocol
type for the following PDU.
In order to be fully interoperable with stations that use this
encoding, Frame Relay stations may recognize the NLPID value of 0xCE
and interpret the following two byte Ethertype. It is never
necessary to generate this encapsulation format only to properly
interpret it's meaning.
For example, IP encapsulated with this NLPID value will have the
following format:
+-----------------------+-----------------------+
|Q.922 Address |
+-----------------------+-----------------------+
|Control (UI) 0x03 | NLPID 0xCE |
+-----------------------+-----------------------+
|Ethertype [7] 0x0800 |
+-----------------------+-----------------------+
| IP Packet |
| . |
| . |
+-----------------------+-----------------------+
16. Appendix
List of Known NLPIDs
0x00 Null Network Layer or Inactive Set
(not used with Frame Relay)
0x80 SNAP
0x81 ISO CLNP
0x82 ISO ESIS
0x83 ISO ISIS
0xCC Internet IP
0xCE EtherType - unofficial temporary use
List of PIDs of OUI 00-80-C2
with preserved FCS w/o preserved FCS Media
------------------ ----------------- --------------
0x00-01 0x00-07 802.3/Ethernet
0x00-02 0x00-08 802.4
0x00-03 0x00-09 802.5
0x00-04 0x00-0A FDDI
0x00-05 0x00-0B 802.6
0x00-0D Fragments
0x00-0E BPDUs
17. References
[1] International Telegraph and Telephone Consultative Committee,
"ISDN Data Link Layer Specification for Frame Mode Bearer
Services", CCITT Recommendation Q.922, 19 April 1991 .
[2] American National Standard For Telecommunications - Integrated
Services Digital Network - Core Aspects of Frame Protocol for
Use with Frame Relay Bearer Service, ANSI T1.618-1991, 18 June
1991.
[3] Information technology - Telecommunications and Information
Exchange between systems - Protocol Identification in the
Network Layer, ISO/IEC TR 9577: 1990 (E) 1990-10-15.
[4] Baker, Fred, "Point to Point Protocol Extensions for Bridging",
Point to Point Working Group, RFC-1220, April 1991.
[5] International Standard, Information Processing Systems - Local
Area Networks - Logical Link Control, ISO 8802-2: 1989 (E), IEEE
Std 802.2-1989, 1989-12-31.
[6] Plummer, David C., An Ethernet Address Resolution Protocol",
RFC-826, November 1982.
[7] Reynolds, J. and Postel, J., "Assigned Numbers", RFC-1060, ISI,
March 1990.
[8] Finlayson, Mann, Mogul, Theimer, "A Reverse Address Resolution
Protocol", RFC-903, Stanford University, June 1984.
[9] Postel, J. and Reynolds, J., "A Standard for the Transmission of
IP Datagrams over IEEE 802 Networks", RFC-1042, ISI, February
1988.
[10] IEEE, "IEEE Standard for Local and Metropolitan Area Networks:
Overview and architecture", IEEE Standards 802-1990.
[11] Bradley, T., and C. Brown, "Inverse Address Resolution
Protocol", RFC-1293, Wellfleet Communications, Inc., January
1992.
18. Security Considerations
Security issues are not addressed in this memo.
19. Authors' Addresses
Terry Bradley
Wellfleet Communications, Inc.
15 Crosby Drive
Bedford, MA 01730
Phone: (617) 275-2400
Email: tbradley@wellfleet.com
Caralyn Brown
Wellfleet Communications, Inc.
15 Crosby Drive
Bedford, MA 01730
Phone: (617) 275-2400
Email: cbrown@wellfleet.com
Andrew G. Malis
BBN Communications
150 CambridgePark Drive
Cambridge, MA 02140
Phone: (617) 873-3419
Email: malis@bbn.com
AI ELI5
On this page
PROBLEMFrame Relay carries only frames, with nothing to say which protocol is inside, so devices sharing a Frame Relay network could not carry IP, bridged LAN traffic and other protocols together.
MECHANISMEach frame carries a Q.922 address, a control byte, an optional pad and a Network Level Protocol ID (NLPID). The NLPID names the protocol, or points to a SNAP header for protocols that lack one. The RFC also covers bridged LAN frames, fragmentation, parameter negotiation and ARP.
STATUS TODAYRFC 1294 was a Proposed Standard that has been obsoleted by RFC 1490, and later by RFC 2427.
§1Scope and Frame Relay Basics
RFC 1294 is about the end stations (DTEs) attached to a public or private Frame Relay network. It does not describe the network's own equipment (DCEs) except where a DTE must react to it.
The network offers virtual circuits, each identified at an interface by a Data Link Connection Identifier (DLCI). In most cases a DLCI has only local meaning at each interface. The rules apply to both switched and permanent virtual circuits.
The abstract adds a safety rule: a system that supports this encapsulation and others must know in advance which virtual circuits carry which one, and use this encapsulation only on circuits explicitly configured for it.
Frame Relay gives you numbered lanes between computers. This document is about how to pack different kinds of mail so each lane's receiver knows what it got.
§2The Frame Format and NLPID
Every packet goes inside a Q.922 Annex A frame. After the Q.922 address, which is normally two octets and holds a 10-bit DLCI, comes a control field. It is 0x03 (UI) unless negotiated otherwise. An optional pad of zero or more 0x00 octets can align the rest of the frame. Then comes the NLPID, followed by the data and the frame check sequence.
The NLPID tells the receiver which protocol follows. Known values include 0x80 (SNAP), 0x81 (ISO CLNP), 0x82 (ISO ESIS), 0x83 (ISO ISIS) and 0xCC (IP). A value of 0x00 is invalid because it cannot be told apart from padding.
Protocols with no assigned NLPID use 0x80 and a SNAP header: a three-octet Organizationally Unique Identifier (OUI) and a Protocol Identifier (PID). OUI 0x00-00-00 means the PID is an EtherType.
No maximum frame size is commonly implemented, but a network must support at least 262 octets, and a DTE must allow its maximum to be configured. The minimum is five octets between the flags.
Every frame starts with a little label that says what kind of thing is inside, so the receiver knows which program should open it.
- 0x00Null / inactive set — Not used with Frame Relay.
- 0x80SNAP — A SNAP header follows.
- 0x81ISO CLNP — ISO connectionless network protocol.
- 0x82ISO ESIS — ISO end system to intermediate system routing.
- 0x83ISO ISIS — ISO intermediate system routing.
- 0xCCInternet IP — IP datagram follows directly.
- 0xCEEtherType — Unofficial temporary use.
§3Routed and Bridged Traffic
The RFC separates routed packets from bridged packets. A routed packet whose protocol has an assigned NLPID uses it directly, which saves 48 bits. Others use 0x80, OUI 0x00-00-00 and their EtherType. For IP the RFC chooses the NLPID 0xCC: it is 48 bits shorter than the SNAP form and is consistent with IP over X.25. For ISO CLNP the NLPID is 0x81. SNAP is used only when no NLPID exists for a protocol.
Bridged packets always use NLPID 0x80 with OUI 0x00-80-C2 (the 802.1 organization). The PID says which LAN type the MAC frame came from and whether the original LAN FCS is kept: for example 0x00-01 or 0x00-07 for 802.3/Ethernet, depending on whether the FCS is preserved. PID 0x00-0E marks bridge protocol data units.
A Frame Relay bridge must flood, forward and filter. Flooding means sending through every relevant circuit. Bridges must also learn dynamically which destination MAC addresses sit behind which circuit, which is why bridged packets carry the encapsulation of section 7.
Some packages are sent by following the road signs (routed) and some are copied from one neighborhood to another (bridged). Each type gets a different kind of label.
§4Negotiation and Fragmentation
Stations may support the XID exchange of Q.922 Appendix III to negotiate the maximum frame size N201, the retransmission timer T200 and the maximum number of outstanding I frames K. Without it, these are configured by agreement or default to the Q.922 values: N201 of 260 octets, T200 of 1.5 seconds, and K of 3, 7, 32 or 40 depending on link speed. A station can decline acknowledged-mode operation by giving K as zero, and a station that receives a smaller remote frame size must reduce its own before replying.
Because a network may allow frames as small as 262 octets, the RFC defines fragmentation that is limited to the Frame Relay boundary, unlike IP fragmentation. Fragments use SNAP with OUI 0x00-80-C2 and PID 0x00-0D, followed by a two-octet sequence number, a final bit, 4 reserved bits set to zero and an 11-bit offset counted in units of 32 bytes. Fragments must be sent in order without interruption by other traffic for the same circuit.
An end station must be able to reassemble up to 2K octets, and 8K is suggested. If any fragment is corrupted or missing, the whole message is dropped and the upper layer must retransmit.
If a package is too big for the road, it is cut into pieces numbered in order and the receiver glues them back together. If a piece is lost, the whole package is thrown away.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
SEQUENCE | Sequence number | 2 bytes | Incremented each time a new complete message is fragmented. | |
FINAL | F | 1 bits | 1 on the last fragment, 0 on all others. | |
RESERVED | RSVD | 4 bits | 0 | Reserved, must be 0. |
OFFSET | Offset | 11 bits | Logical offset of this fragment in bytes divided by 32. |
§5Address Resolution: ARP, RARP and Inverse ARP
ARP, defined in RFC 826, is carried in a SNAP frame with OUI 0x00-00-00 and PID 0x0806. The hardware type for Frame Relay is 15, and the hardware addresses are Q.922 addresses with the C/R, FECN, BECN and DE bits set to zero.
The catch is that DLCIs are usually local, so a station does not know its own. The network rewrites the DLCI in the header as the frame crosses it, so the receiver sees the DLCI that stands for the sender. The receiving station therefore copies the address from the Frame Relay header into the source hardware address field, as its example with stations A, B and C shows. Reverse ARP (RFC 903) works the same way.
Without multicast, ARP can be simulated by sending a copy of the request through each relevant circuit. Because that broadcast is inefficient, Inverse ARP (RFC 1293) resolves a protocol address when the DLCI is already known, using operation code 8 for the request.
Your own lane number is not printed on your side of the road, so the receiver reads the sender's lane number from the envelope and writes it into the question.
§6Backward Compatibility
Some existing stations use NLPID 0xCE as an escape to Ethernet packet types. The RFC includes this only for completeness and adds no new requirement: a station may recognize 0xCE and read the following two-byte EtherType, but it never needs to generate that format. IP sent this way carries EtherType 0x0800.
The RFC leaves end-to-end negotiation of configuration and service parameters for future study. It states that security issues are not addressed.
Some older devices use a different kind of label. This document tells newer ones how to read it, but not to write it.
§7References
[3]ISO/IEC TR 9577: 1990, Protocol Identification in the Network Layer.
[6]RFC 826Plummer, David C., "An Ethernet Address Resolution Protocol", RFC 826, November 1982.November 1982[7]RFC 1060Reynolds, J. and Postel, J., "Assigned Numbers", RFC 1060, March 1990.March 1990[8]RFC 903Finlayson, Mann, Mogul, Theimer, "A Reverse Address Resolution Protocol", RFC 903, June 1984.June 1984[9]RFC 1042Postel, J. and Reynolds, J., "A Standard for the Transmission of IP Datagrams over IEEE 802 Networks", RFC 1042, February 1988.February 1988[11]RFC 1293Bradley, T., and C. Brown, "Inverse Address Resolution Protocol", RFC 1293, January 1992.January 1992